In a conventional cooling system for an internal combustion engine, the coolant is circulated by a circulating pump through the engine block, through the cylinder heads, eventually through the intake manifold. In an automobile, or other land vehicle, the coolant flows from the manifold through a radiator. Most marine engines do not have a radiator. In a marine engine having an open loop cooling system, sea water supplied by a sea water pump is used to directly cool the engine and is then discharged overboard.
Other marine engines, particularly those designed for use in salt water, have closed loop cooling systems. In a closed loop cooling system, an engine coolant circulates through the engine and then through the shell side of a shell and tube heat exchanger. Raw seawater from an ocean or lake is pumped through the tube side of the heat exchanger, so that excess heat from the engine coolant is transferred to the raw sea water stream. The sea water is then discharged in the ocean or lake. Thus, raw sea water never passes through the coolant passages of the engine, thus reducing corrosion within the system, which is especially important when operating in salt water.
In closed loop cooling systems, the flow of engine coolant through the heat exchanger is controlled by a thermostat, such as a thermostatically activated valve. The thermostat in a marine engine with a closed loop cooling system is usually located on the intake manifold. When the engine and engine coolant are cold, the thermostat is closed so that coolant does not pass through the heat exchanger. With the thermostat closed, a small portion of engine coolant flows through a bypass and returns a limited amount of coolant to the circulating pump, so that there is a sufficient engine coolant flow through the engine block while the system is warming up. While a limited amount of engine coolant is adequate when the engine warms up slowly, problems can occasionally exist if the operator runs the engine wide open before the engine and coolant have warmed up. Under these conditions, coolant flow through the engine bypass before the thermostat opens might not provide sufficient cooling. This can be critical because wide open operation can quickly lead to hot spots in an engine.
It can be appreciated that an improved thermostat that allows increased flow of coolant before the thermostat opens is desirable in a marine engine having a closed loop cooling system.